Linux does not have a single source of truth. Yes, there is the tree of Torvalds, but thats just for the mainline development. Every company developing Linux has their own trees. They may merge commits off the Torvalds tree and vice versa, but most of the time those developments have different goals (ie Ubuntu vs embedded car media system). GitHub is too much tied to one source of truth.

Pull Requests. Githubs way of receiving contributions is via Pull Requests. For a development with lots of different contributors and maintainers, this does not work. A contribution to the graphics system goes through at least to levels of maintainers that provide feedback, before it goes to Linus for final merge. Having all contributions for all parts in one place would be too much.